JPH1151797A - Apparatus and method for measurement of heat-loss pressure - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a heat-loss pressure gage which enhances the accuracy of the measurement of a pressure in a range from a low pressure up to a high pressure. SOLUTION: A Pirani gage is provided with a small-diameter-wire detecting element 12, a small-diameter-wire correction element 14 which is situated on the same face as it and two heat-conductive plates 16 which are separated from the elements and which are parallel and flat. The physical size, the thermal characteristic and the resistance characteristic of the detecting element 12, the correction element 14 and their coupling element are identical. The coupling element comprises a large heat-conductive property to a uniform temperature region, and both elements are placed in an identical vacuum environment. A DC heating current is used only for the detecting element 12. A comparatively small AC signal is used to detect the balance of a bridge. Then, by a simple three-dimensional pressure correction system, a correction is performed precisely, and the acquisition of calibration data is simplified.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat loss gauge using gas conduction for measuring a pressure ranging from a very low pressure to a pressure higher than the atmospheric pressure.

[0002]

2. Description of the Related Art Since the thermal conductivity of a gas is a function of the pressure of the gas, the thermal conductivity from a heated detector element to the gas is measured under appropriate conditions with appropriate calibration, and is measured. Can be used to determine pressure. This principle is also used in the well-known Pirani gauge (shown in FIGS. 1a and 1b).
In this device, heat loss is measured by a network of Wheatstone bridges, which warms the sensing element and measures its resistance.

In the Pirani gauge shown in FIG. 1a, the pressure sensor is coupled to one side of a Wheatstone bridge and consists of a temperature-sensitive resistor RS. RS is a temperature sensitive resistor that is typically designed so that the temperature rise due to current i ₂ is negligible. R3 and R4 are typically fixed resistors. RS and typically R2 are exposed to the vacuum environment to be pressure measured. FIG. 1b shows another bridge configuration.

The Pirani gauge is disclosed in US Pat. No. 3,580,008.
It operates with a constant current i ₁ (as shown in No. 1) or with a constant voltage across RS. In these methods, an electrical imbalance of the bridge occurs, which reflects the pressure of the gas. Pirani gauge is disclosed in US Pat.
It also operates with a constant resistance value RS (as shown in 38387). In this mode, the rate at which energy is supplied changes with the pressure change of the gas, so the rate of change of the supplied energy reflects the pressure change of the gas. Although each operation has its advantages and disadvantages, the following discussion describes the invariant resistance method and the configuration shown in FIG. 1a.

The voltage VB is automatically controlled so as to fix the voltage difference between A and C in FIG. A to C
If the potential drop to is zero, the bridge is said to be in bridge balance. The following conditions exist for bridge balance:

[0006]

(Equation 1)

[0007]

(Equation 2)

[0008]

(Equation 3)

[0009]

(Equation 4)

[0010] By dividing equation 3 by equation 4 and using equations 1 and 2,

[0011]

(Equation 5)

And here

[0013]

(Equation 6)

## EQU1 ## Therefore, in the bridge balance, RS represents the coefficient β of R2.

In order for RS to be able to assume a steady state at a certain pressure, equation (7) must be satisfied:

[0016]

(Equation 7)

The conventional Pirani gauge has an unknown pressure P
_A series of known pressures are calibrated to determine the relationship between _x and the power escaping to the gas or more conveniently to the bridge voltage. The unknown gas pressure P _x is then directly determined from the power escaping to the gas or the power related to the bridge voltage at the bridge balance, assuming that the losses from both ends and the losses due to radiation are constant.

Since the Pirani gauges are designed to be versatile, relatively simple and inexpensive, there is a long-standing need to use these gauges as a substitute for more expensive gauges such as capacitance manometers and ionization gauges. was there. However, existing designs leave many problems for precise pressure measurements, especially at low pressures.

Prior to 1977, the Pirani gauge had an upper pressure limit of about 20 Torr, due to the fact that under high pressure, the heat transfer of the gas was substantially independent of the pressure in the macro-sized device. One of the present inventors assisted in the development of a Convectron (TM) gauge manufactured and sold by the assignee (Granville-Phillips, Boulder, CO) since 1977.
To increase the sensitivity from 0 to 1000 Torr, convective cooling of the sensing element is used. Hundreds of thousands of Convectron gauges were sold worldwide. Recently, imitations have been on the market.

Although the convexron gauge met certain needs, it had several disadvantages. In order to secure enough space for convection, the inside size must be large. Therefore, the convexron gauge is relatively large. Since convection depends on gravity, pressure measurement under high pressure depends on the orientation of the sensor axis. Also, the pressure range for cooling by gas conduction does not overlap exactly with the pressure range for convection cooling, so the sensitivity of a convextron gauge is limited between about 20 and 200 torr.

In order to avoid these problems, microminiature Pirani sensors have been developed, where the distance from the sensing element to the wall is on the order of a few microns instead of the large one used previously, eg 0.5 inch. See, for example, U.S. Patent No. 4,682,503 to Higashi et al.
No. 5,347,869 to Shie et al. J. J. Alvesteffer et al. Va
c. Sci. Technol. An article in A13 (6) November / December 1995 describes the latest research on the Pirani gauge. By shortening the distance from the detection element to the wall as described above, the heat conduction can be made to depend on the pressure even at a pressure higher than the atmospheric pressure. Thus, such a microscopic sensor has good sensitivity ranging from low pressure to above atmospheric pressure and works in any orientation.

There are various problems when trying to develop a micro gauge. Although microsensors have good sensitivity regardless of orientation over a wide range of pressures, their design is extremely complex, and their manufacture requires many hundreds of thousands of dollars using very specialized equipment. Requires elaborate manufacturing processes.

Micro sensors, like macro-sized sensors, suffer from ambient temperature errors. All of the heat loss terms in equation (7) depend on the ambient temperature and also on the temperature of the sensing element at a given pressure. Therefore,
Attempting to measure pressure using a Pirani gauge without temperature compensation will confuse the measurement results due to ambient temperature changes and power losses independent of pressure. Current Pirani gauges attempt to correct for errors caused by changes in ambient temperature. A widely used means for correcting such errors is to connect a temperature sensitive correction element RC in series with a fixed resistor R and R2 as shown in FIGS. 1a and 1b.
Instead of using it.

GB 2105047A discloses the use of additional resistors to form a voltage divider. J.
H. "Pressure Measurement in Vacuu by Leck
m ”(Chapman and Hall, London, 1664)
Page 8 states that Hale used R2 in 1911 for the Pirani gauge with the same material and the same physical size as RS. R2 was sealed in its own vacuum environment and placed near RS. If the pressures at R2 and RS are the same, the temperature correction is very good. However, at other pressures, temperature compensation by this means is not very effective.

R2 has traditionally been placed in the same vacuum environment as RS to avoid the extra expense and complexity of vacuum sealing R2 in a separate glass sphere. By making R2 have a relatively large thermal mass and heat loss, R2
The effect of warming itself can be ignored. Leck recommends that R2 be "two-element, one copper and the other nichrome, so that the overall temperature coefficient (of R2) matches that of the Pirani element itself (RS)". I have. According to Leck, this method of temperature compensation is a METR from Edwards High Vacuum of Great Britain, a UK company.
It has been used for OVAC brand gauges. A similar temperature compensation method is used for CONVECTRON brand gauges.

However, (Temperature coefficient of resistance is different 2
This technique is useful only in a narrow pressure range (using more than one type of material for R2 to approximate the temperature coefficient of RS). In fact, this correction is only accurate at one or at most several temperatures, as in US Pat. No. 4,541,286 which describes a temperature correction in this way in a Pirani gauge. The inventors have also discovered that configurations using objects of large thermal mass greatly increase the response time of the gauge to sudden changes in ambient temperature.

Also, extensive computer simulations have shown that making the temperature coefficients of RS and R2 the same as recommended by Leck and performed in the prior art does not provide completely accurate temperature correction. The present inventors have also found out. Further, it was found that at a pressure of about 5 × 10 ⁻³ Torr or less, the loss from the end of the detecting element was larger than the sum of all other losses.
The relative loss components (radiation loss, terminal loss, and gas loss components in the total loss) determined by this study are as shown in the graph of FIG. At 1.times.10.sup.- ⁵ torr, terminal losses are more than 1000 times greater than gas losses and radiation losses are about 100 times greater than gas losses.

Thus, the effect of temperature changes in conventional Pirani gauges is particularly problematic at very low pressures where gas conduction losses are very low. Conventional heat loss gauges cannot accurately measure very low pressures, for example, 1 × 10 ⁻⁵ Torr. The inventor has discovered that this limit is the result of the terminal losses in the sensing element not being kept sufficiently constant when the ambient temperature changes. Alvestipher-type Pirani gauges are capable of displaying pressure in a range such as 10 ^-5 Torr, but cannot provide accurate readings in this range. For example, with a typical Pirani gauge, if the end loss is not kept at 1/5000, at 1 × 10 ^-5 Torr, the pressure reading will be 50-100% less.

The following analysis shows why conventional designs fail to adequately compensate for ambient temperature changes at low pressures. In verifying the prior art, the problem will be described by conveniently using an example of a gauge in which the space between the detection element and the wall is relatively large. It should be understood that similar problems are found in the complex structure of micro gauges on the order of a few microns from the sensing element to the wall.

FIG. 3 shows one element 302 of a conventional Pirani gauge.
This gauge uses a small diameter wire sensing element 304 and a compensating element 303. Those familiar with the design of the Pirani gauge will note that the elements of FIG. 3 are not drawn to scale for ease of explanation and understanding. Typically, a small diameter wire sensing element 30
4 is also electrically and thermally coupled to larger electrical connectors 306 and 307, which are thermally coupled to larger support structures 308 and 309. T _AL
Is the temperature of the support structure 308 at the left end of the detection element 304 at a certain time t, and T _AR is the temperature of the support structure 309 at the right end of the detection element 304. T _SL and T _SR are respectively connected to the left detection element connector 306 and the right detection element connector 307.
Temperature. _Let T _CL and T _{CR be} the temperatures of the left correction element connector 310 and the right correction element connector 311 respectively. TXL and TXR are each connected to the left detecting element connector 3.
06 and the temperature at a point ΔX away from the right detection element connector 307. In conventional designs, it has been explicitly assumed that these temperatures are all the same. However, the inventors have found that even seemingly negligible differences have a significant effect on the accuracy of measurements at low pressures.

To better understand what is required in temperature compensation, it is important to note several things.

(1) At low pressures, the temperature of RC is mainly determined by the heat exchange between the correction element and its coupling element.
This is because radiation and gas conduction are very inefficient to transfer heat from the correction element to its surroundings at ambient temperature and low pressure compared to heat conduction from both ends of the correction element. Thus, at low pressure, as shown in Equation 8, the temperature of the correction element will be very close to the average of the connector temperatures across the correction element.

[0033]

(Equation 8)

(2) The temperature of the electrically heated detection element changes from both ends to the center, and increases as the distance of the cooler from the support element increases. The present inventor simulated the temperature distribution of the detection element using finite element analysis. When the same temperature coefficient of RS and RC of the resistor, even if the temperature T _n varies constant pressure, together with the average temperature T _AVG correcting elements RC bridge balance in any part n of the detection element, the difference [Delta] T _n = T _n - It changes so that _TAVG becomes constant. The difference ΔT _n is a function of R and β, _where R = R2−RC.

(3) According to Equation 5, the resistance RS of the detection element in the bridge balance is maintained at β times the resistance of R2. As the ambient temperature increases, the temperature of the correction element connector also increases, so that the temperature and resistance of RC change according to Eq. Any increase in the temperature of RC and the resulting increase in resistance will cause an increase in the temperature and resistance of all parts of the RS in bridge balance.

(4) The power lost from both ends of the detection element depends on the temperature gradient γ between both ends of the detection element as shown in Equation 9:

[0037]

(Equation 9)

Where k is a constant,

[0039]

(Equation 10)

[0040]

[Equation 11]

Is as follows. If γ _L or γ _R changes for any reason, the terminal losses will also change and the pressure reading will be inaccurate.

To better understand the significant deficiencies of the prior art temperature compensation at low pressure, assume that T _AR has increased only slightly from steady state, eg, the local ambient temperature of the right support structure. _TAL remains unchanged. Assuming that T _AL does not change, both T _CL and T _SL are unchanged. However, an increase in T _AR increases T _CR because heat conduction occurs through the coupling element. Therefore, T _AVG also increases.
An increase in T _AVG also increases T _XL and T _{XR in the} bridge balance, causing changes in γ _L and γ _R. Changes in γ _L and γ _R change the terminal loss term in Equation 7, causing errors in pressure measurements that depend on the magnitude of the change in γ _L and γ _R.

The inventor has determined that the terminal loss of the sensing element does not always change when the ambient temperature changes unless T _AL changes substantially in the same manner as T _AR . Conventional Pirani gauges were not designed to keep T _AL equal to T _AR to the extent necessary to accurately measure low pressures.

To understand another significant deficiency in conventional temperature compensation, the ambient temperature rises from steady state and T _AL =
Assume T _AR . Further, it is assumed that the length of the detection element is the same, but that the right correction element connector is longer than the left one, as in the Pirani gauge conventionally used. Therefore, T _SL = T _SR, but T _CR changes later than T _CL because of the difference in length. During this delay when T _CL ≠ T _CR
AVG is changed, T _XL and T _XR changes at bridge balance. Therefore, during this delay, γ _L and γ _R continuously change, causing an error in pressure display at low pressure.

The inventor has determined that the loss from the end of the sensing element is not invariant if the ambient temperature changes, unless the sensing element connector and the correction element connector are physically the same size and have the same thermal characteristics. . The conventional Pirani gauge has not been specifically designed so that the detection element connector and the correction element connector have the same physical size and the same thermal characteristics.

Another important defect is due to the mass difference between the correction element and the detection element (as discovered by the inventor). Assume, as is typically the case, that the mass of the correction element is substantially greater than that of the detection element. In a conventional Pirani gauge, it is usual to make the correction element larger than the detection element. This creates a relatively large heat loss path to the periphery of the correction element and disperses the generation of heat due to power loss in RC. Let us assume that the ambient temperature rises from the steady state and that T _AL = T _AR . Thus, the time it takes for the correction element to reach the new steady state temperature is longer than the time it takes for T _SL and T _SR to reach the new steady state temperature. Has been). During this time, T _AVG changes continuously, and T _XL and T
Change _XR continuously. Therefore, γ _L and γ _R also change during this lag time, and the terminal loss of the detection element is not constant, causing an error in measurement of low pressure.

A similar problem arises even when the correction element is designed to change the temperature at a different speed from the detection element as the ambient temperature changes due to bridge balance. Conventional designs such as Alvestipher-type devices suffer from this deficiency.

Research has shown that unless the correction element is designed to change temperature at the same rate as the detection element, the terminal losses of the detection element continue to change long after the ambient temperature has settled to a new value. The inventor has discovered. However, conventional Pirani gauges have not been designed to meet this requirement.

By using a correction element RC having the same temperature coefficient of resistance as that of the detection element in series with the resistance R which does not respond to temperature in place of R2, gas loss which changes as a temperature difference between the detection element and its surroundings can be obtained. It has long been known that it is possible to make temperature corrections for terminal losses. This temperature compensation method has been used for many years on CONVECTRON gauges, and has also been used on Alvestipher gauges.

In this temperature correction method, if (1) the temperature coefficient of the resistance of the detection element and the temperature coefficient of the resistance of the correction element are the same, and (2) the change in the resistance of the detection element increases with the change in the resistance of the correction element. (3) It is stated that the temperature of the detecting element rises with a change in the ambient temperature. Meeting these two assumptions is, of course, very ideal. This is because satisfying these assumptions ensures that the temperature difference between the warmed sensing element and the surrounding wall at ambient temperature is constant even when the ambient temperature changes.

However, the present inventor has discovered that the conventional gauge which uses the fixed resistor R and the temperature-sensitive resistor RC in series in place of R2 can perform only partial temperature correction as described below.

In FIG. 1a, it is assumed that R2 comprises a temperature-sensitive correction element RC and a temperature-independent resistor R. Therefore

[0053]

(Equation 12)

Is as follows. Thus, Equation 5 above derived for bridge balance may be written as:

[0055]

(Equation 13)

Here, β is defined by the above equation (6).

[0057] Further, assume that the ambient temperature of the gauge environment when equal to T _1, the detection device operates at T _S1, the correction element operates at T _C1. Therefore, if

[0058]

[Equation 14]

Then, Equation 13 can be expressed as follows.

[0060]

(Equation 15)

Here, RS (T ₁ ) is the resistance of the detection element at the temperature T ₁ , and α _S is the temperature coefficient of the resistance RS at the temperature T ₁ . RC (T ₁ ) is the resistance of the correction element at the temperature T ₁ , and α _C is the temperature coefficient of the resistance RC at the temperature T ₁ . Thus, if T _AMBIENT = T ₂ , Equation 13 may be written as:

[0062]

(Equation 16)

Solving equation 15 for T _S1 gives

[0064]

[Equation 17]

And solving Equation 16 for T _S2 ,

[0066]

(Equation 18)

Is obtained. By subtracting Equation 17 from Equation 18, the temperature change ΔT of the detection element RS when the ambient temperature changes from T ₁ to T ₂ can be found. Therefore

[0068]

[Equation 19]

Is as follows. Note that an effective correction element is designed such that its temperature approaches the ambient temperature. So for a good approximation

[0070]

(Equation 20)

Is obtained. Therefore, Equation 19 can be expressed as follows.

[0072]

(Equation 21)

From equation 21, the temperature change ΔT of the detecting element RS
It is evident that equal only when the following changes in ambient temperature T ₂ -T _1.

[0074]

(Equation 22)

As shown in FIG. 1a, the conventional gauge using a temperature-sensitive correction element RC in series with the fixed resistor R instead of R2 can provide only a partial temperature correction depending on the selection of β. A commercially available gauge of the design described by Alvestifer et al., The latest product of the Pirani gauge known to the present inventor, does not satisfy Equation 22.

As a third problem with the conventional gauge design, the inventor has discovered that the degree of power loss in R2 has a disadvantageous effect on accuracy. In the conventional Pirani gauge configured as shown in FIG. 1A, a pressure-dependent current flows through RS, similarly to the bridge balance correction element. When configured as in FIG. 1b, a pressure dependent voltage is equally applied to RS and R2 at bridge balance. Of course, the current dependent on the pressure flowing through R2 causes the temperature of RC to rise above ambient temperature, and this rise varies with pressure.

Conventional Pirani gauges typically use correction elements that are physically much larger than detection elements. This is to dissipate the heat, thereby preventing the extra temperature rise of the correction element. As described above, if the sizes of the detection element and the correction element are different, a measurement error occurs when the ambient temperature changes.

A fourth problem is that in the conventional Pirani gauge, when the ambient temperature changes, the pressure display fluctuates at a low pressure. Conventional Pirani gauges use various components to maintain the power lost from the sensing element even when the ambient temperature changes. For example, as in US Pat. No. 4,682,503, thermoelectric cooling is used for ambient temperature control, thereby minimizing ambient temperature changes.

The device described in US Pat. No. 4,541,286 incorporates a thermally sensitive element near the side of the correction element of the bridge (in fact, for commercial use this is outside the vacuum enclosure). Are joined). Alvestifer et al. Have additional elements (designed as R4)
Is added to the bridge to compensate for the fact that the temperature coefficient of the resistance of the sensing element at operating temperature differs slightly from that of the correction element at ambient temperature. Such conventional hardware can remove errors caused by changes in ambient temperature to some extent, but cannot remove all of them. Therefore, conventional Pirani gauges have significant fluctuations in pressure readings at low pressures when the ambient temperature changes.

Another conventional system described in US Pat. No. 5,608,168 relates various electrical measurements (or an approximation) of the bridge, determines the value of the temperature-dependent resistance or temperature, and measures this parameter with the pressure measurement. Consider in. However, this system adds to the complexity of having to measure temperature and other values.

Accordingly, there is a need for an improved Pirani-type gauge that overcomes these problems.

[0082]

SUMMARY OF THE INVENTION It is a general object of the present invention to provide an improved apparatus and method of operation for heat loss pressure measurement.

Another general object is to provide an improved Pirani-type pressure gauge having a circuit for passing a heating current through a temperature sensing element and not passing a heating current through an associated correction element.

Further, a more specific object of the present invention is to
An improved Pirani-type correction element wherein the correction element is substantially the same physical size as the detection element, is in the same plane as the detection element, but is spaced from the detection element, and is made of the same material as the detection element. Is to provide a pressure gauge.

It is a further object of the present invention to provide an improved heat loss pressure gauge in which a heat conducting element is positioned near the sensing element such that the temperature is the same throughout the sensing element.

Further, there is provided an improved heat loss pressure gauge further including a means for ideally keeping a distance between the detecting element and the heat conducting element in order to correct expansion and contraction of the detecting element in accordance with a change in ambient temperature. It is an object of the present invention.

It is a further object of the present invention to provide an improved pressure gauge in which the physical size and thermal conductivity of the heat flow path of the sensing element is substantially the same as that of the correction element.

It is a further object of the present invention to provide an improved heat loss pressure gauge in which the voltage and current of the temperature compensating element are independent of pressure.

It is a further object of the present invention that the heating means produces a constant difference between the resistance of the detection element and the resistance of the correction element at ambient temperature without applying a heating voltage or current to the correction element. To provide an improved heat loss pressure gauge.

It is a further object of the present invention to provide an improved heat loss pressure gauge using a DC heating voltage.

A further general object of the present invention is to provide an improved temperature compensation method for use in Pirani-type gauges.

A more specific object of the present invention is a Pirani gauge in which the voltage and current values of the sensing element are continuously recorded for a series of pressures and ambient temperatures and can define three or more dimensional calibration surfaces. It is to provide an improved method of temperature compensation to be used.

A further object of the invention is, for example, pressure,
It is an object of the present invention to provide an improved method of temperature compensation used in Pirani gauges, in which accurate calibration is obtained by using a model using only three dimensions such as voltage and current.

A further object of the present invention is to recognize the equation P = f (V, I) which approximates the collected calibration data, where P is the pressure corresponding to voltage V and current I, and Is to provide an improved method of temperature compensation for use in Pirani gauges, where accurate calibration is performed by using in the calculation of pressure during operation.

A further object of the present invention is to recognize the equation P = g (W, R) which approximates the calibration data collected, where P is the pressure corresponding to power W and resistance R, and Is to provide an improved method of temperature compensation for use in Pirani gauges, where accurate calibration is performed by using in the calculation of pressure during operation.

A further object of the present invention is to use a bridge circuit using a detection element, a correction element and two fixed resistors, and to multiply the correction element at a given temperature by the temperature coefficient of the correction element for that temperature. The resistance multiplied by the resistance of one bridge fixed resistor is to provide a heat loss pressure gauge equal to the temperature coefficient of the sensing element for that temperature, the resistance of the sensing element multiplied by the other bridge fixed resistor. .

[0097]

SUMMARY OF THE INVENTION Various improvements are provided in the present invention that work together to significantly improve the accuracy of pressure measurements at low, medium and high pressures, thereby providing accurate pressure measurement ranges with a single gauge. These objectives are accomplished by extending from low pressure to high pressure.

As a first improvement, the small diameter wire sensing element is spaced apart in the same plane as the small diameter wire compensating element, along with two parallel flat thermally conductive plates comprising the sensing element and the compensating element. From each other at 15 microns. As a result, the present inventor has realized high relative sensitivity with simple geometry without relying on convection. The extreme complexity and cost of the microminiature Pirani gauge design and some disadvantages of convective cooling of the sensing element are avoided at the same time.

This extremely simple, small and inexpensive measuring means produces results up to about the same high pressures as have been obtained with very complex microminiature Pirani gauges or larger, more position sensitive convection cooled Pirani gauges. be able to. Surprisingly, this improvement requires only 3% of the detector element of the micro Alvestipher gauge. The correction element of the new device has a volume of less than 0.5% of the Alvestipher-type element.

The present invention also improves temperature correction. The accuracy of the pressure measurement at low pressure is significantly improved by better keeping the temperature gradient γ (see equations 10 and 11) at both ends of the sensing element. Consistency of γ is achieved by simultaneously doing the following: 1 Use the same physical size, thermal and resistance characteristics for the sensing and compensating elements.

2. The same element having the same physical size, thermal characteristics and resistance characteristics is used as the coupling element between the detection element and the correction element.

3 Resistive coupling elements with substantially the same high thermal conductivity are used in a uniform temperature zone of all coupling elements.

4 The detection element and the correction element are placed in the same vacuum environment.

In the present invention, since the gauge is designed to satisfy the following equation (23), equation (22) is always satisfied.

[0105]

(Equation 23)

Where TA is the ambient temperature and

[0107]

(Equation 24)

[0108]

(Equation 25)

Is as follows.

Another significant improvement can be achieved by making the heating of the correction element negligible. The present inventor has improved the conventional Wheatstone bridge so as to provide independent heating means for the detector element while substantially not heating the other three Wheatstone bridge sides. Therefore, the correction element can be made to have the same physical characteristics and the same size as the detection element. A DC heating current is used, which is limited to the sensing element only. A relatively small AC signal is used for bridge balance detection.

Another performance improvement is achieved by providing a new pressure compensation method that provides accurate pressure readings at all pressures. In particular, an unknown pressure P _X at bridge balance may be simply determined by equation 26 below.

[0112]

(Equation 26)

Here, VS is the pressure drop of the detection element,
IS is the current of the detection element. Characteristics of the formula 26, using three-dimensional curve fitting software, subject to pressure and a series within a temperature range of values of a known pressure P _C and VS _C and IS _C obtained by calibration against ambient temperature Derived from the pair. VS _X and IS _X are measured at the unknown pressure P _X at bridge balance and substituted into Equation 26. afterwards,
P _X is computed using a microprocessor.

[0114]

Thus, the present invention provides significant advances in Pirani gauge accuracy, manufacturing cost, and package size.

[0115]

DETAILED DESCRIPTION OF THE INVENTION The present invention will be described with respect to four improvements made to a conventional Pirani gauge design. In a particularly preferred embodiment, these four improvements are used simultaneously and work together to provide a Pirani gauge with improved performance.

Improvement 1 The first improvement is described with reference to FIGS. 4a and 4b. FIG. 4a shows an improved heat loss gauge (not to scale)
It is a side view of the part 10 of FIG. FIG. 4b shows the line 4b-
It is sectional drawing of the gauge part 10 which follows 4b. As shown in these figures, the small-diameter wire detecting element 12 is placed in the same plane as the small-diameter wire correcting element 14 at a distance d. The size of the distance d is preferably about 0.030 inches, but may be in the range of 0.010 to 0.200 inches. In the vicinity of the detection element 12 and the correction element 14, two parallel plates 16 and 16 'are placed parallel to them.

The parallel plates 16 and 16 ′ are the detector 1
2 and the correction element 14 by a distance S. S is preferably 0.0007 inches, but may be in the range of 0.0002 to 0.002 inches. The detecting element 12 is made of a material such as pure tungsten having a high temperature coefficient of resistance, and may be gold-plated to maintain a constant emissivity.

The diameter of the detecting element 12 is preferably 0.0005 inch, but may be in the range of 0.0001 to 0.002 inch. Although a cylindrical wire shape is preferred, other shapes, such as a ribbon shape, may be used for the detection and correction elements. The length of the detection element 12 is preferably 1 inch, but may be in the range of 0.25 to 3 inches.

The correction element 14 is made of the same material and the same size as the detection element 12, and has the same thermal characteristics and resistance characteristics.

The part 10 of the heat loss gauge may be incorporated in a measuring circuit of the type shown in FIG. 6 in the manner described in detail below.

The parallel plates 16 and 16 'conduct heat, thereby causing a temperature gradient along the detector element 12 and the detector element 1'.
2 and the temperature gradient between the ends of the correction element 14 tends to be equal. Thereby, the present invention can achieve relatively high sensitivity with a simple structure without depending on convection. In this embodiment, the detection element and the correction element having the same size, thermal characteristics, and resistance characteristics are used, and by placing them in the same vacuum environment, the pressure measurement accuracy at low pressure is improved. The use of this design simultaneously avoids the extreme complexity and cost of the microminiature Pirani gauge design and the disadvantages associated with convective cooling of the sensing element. This improvement allows pressure measurements comparable to the atmospheric pressures obtained with very complex microminiature Pirani gauges and larger, position sensitive convection cooled Pirani gauges.

Improvement 2 As a second feature of the present invention, the mounting of the detection element and the correction element is improved. The accuracy of low-pressure pressure measurement is based on the fact that coupling elements with the same physical size, thermal characteristics, and resistance characteristics are used for the detection element and the correction element, and the coupling elements that have substantially the same large thermal conductivity are used as the temperature of all coupling elements. The accuracy of low pressure measurement can be greatly improved by using it in a portion where is uniform.

FIG. 5a is a greatly enlarged cross-sectional view of one end of the gauge part 10, wherein the sensing element 12 is supported by the sensing element connectors 20 and 20 'and is electrically coupled. Correction element 14 is supported by and electrically coupled to correction element connectors 22 and 22 '. FIG. 5a is a cross-sectional view along line 5a-5a of FIG. 4a. Preferably, the same support is present at each end of the gauge part 10 (as shown in FIG. 5a).

The connectors 20, 20 ', 22, 22' are preferably made of platinum ribbon 0.001 inches thick and 0.060 inches wide. Plate 1
Preferably, 6 and 16 'are made of an electrically insulating material having high thermal conductivity such as aluminum nitride.

Alternatively, the detecting element connector and the correcting element connector 20, 20 ', 22, 22' are made of a thin electric insulating film 2 made of tungsten coated with diamond.
It may be electrically insulated from the plates 16, 16 'at 4, 24'. In this case, the plates 16 and 16 'may be made of a material having high thermal conductivity such as tungsten. Preferably, the selected material has a thermal conductivity greater than 0.25 Watt / cm / K.

The plates 16 and 16 'are clamped at one end by simple plate-like metal clamps (not shown). The clamp is connected between the detection element 12 and the correction element 14 by the connectors 20 and 2.
0 ', 22, 22'
Plate 1 until 0, 20 ', 22, 22' are close
Apply sufficient force to 6, 16 '. Therefore, detecting element 1
The distance S between the plate 2 and the plates 16 and 16 ′ is determined by the diameter of the detection element 12 and the thin ribbon-shaped connectors 20, 20 ′, 22 and 2.
It is determined by the thickness of 2 '. This feature of the present invention makes it possible to place a sensing element, which is finer than human hair, at a distance similar to its fineness from two flat surfaces, precisely and very inexpensively and with electrical connection to other circuits. Enable him to be.

The plates 16 and 16 'are zones of substantially uniform temperature, especially when isolated from the outside with a vacuum of minimal thermal conductivity. The thin ribbon connectors 20, 20 ', 22, 22' provide a very large heat transfer to the temperature uniform area in a short path of the same size, thereby keeping the temperature gradient γ at both ends of the sensing element constant To meet some conditions.

As shown in FIG. 5b, the sensing element 12 may be suitably tensioned with a small diameter wire spring 26 during assembly to support the sensing element 12 in the vicinity of its connector 21. Spring 28 is also used to tension correction element 14 as well. The springs 26 and 28 are provided for detecting elements 12 and correcting elements 14 for the plates 16 and 16 'as the ambient temperature changes.
Used to precisely maintain the distance of Plate 1
In order to prevent destruction due to differences in thermal expansion between 6, 16 'and the detection element 12 and the correction element 14, the detection element 12 and the correction element 14 must have sufficient slack. Without the springs 26, 28, this slack changes with ambient temperature, preventing the distance S between the parallel plates 16, 16 'and the detection element 12 and the correction element 14 from being kept constant,
This causes measurement errors.

In the design of an embodiment of the present invention, Equation 22 is partially satisfied by the fact that sensing element 12 and correction element 14 are physically, electrically, and thermally the same. In addition, FIG.
In the embodiment of the present invention, since R3 is the same as R4, β = 1 is secured from Equation 6. Therefore, Equation 22 is always satisfied by this design.

Improvement 3 A third feature of the present invention is a method and an apparatus for exclusively heating the detection element 12. This improvement is shown in FIG. 6, where the Wheatstone bridge 30 has been modified to specifically warm the sensing element 12. In a conventional circuit using a correction element of the same size and the same material as the detection element as in the present invention, the correction element is operated not at the ambient temperature but at the same temperature as the detection element. Therefore, the accuracy of the Pirani gauge improved as described above cannot be achieved by using the conventional heating circuit.

Returning to FIG. 6, the Wheatstone bridge 30 has nodes A, B, C, and D, and nodes B and C
And a detection element 12 having a resistance value RS.
Temperature-independent resistor 15 (resistance value R) and correction element 14
(Resistance value RC) together form a resistor R2. R2 and capacitor 36 are connected in series between nodes C and D.
The resistor 17 has a resistance R4 and is connected between the nodes A and B, and the resistor 19 having a resistance R3 is connected between the nodes A and D. The detection element 12 and the correction element 14 are surrounded by a vacuum environment 34. An AC power supply 38 is connected between nodes B and D, and a frequency selective detector 40 is connected between nodes A and C. DC power supply 32 is connected between nodes B and C to allow current to flow through node B. Controller 42 is coupled to control DC power supply 32 via automatic feedback linkages 46 and 47 and is adapted to receive voltage detection from frequency selective detector 40 for that purpose.

The vacuum environment 34 surrounds a part 10 of the gauge (described above in FIGS. 4a and 4b), which consists of the detection element 12, the correction element 14 and the plates 16, 16 '. Also, the assembly method described with reference to FIGS. 5a and 5b is preferably used for the circuit of FIG. The connectors 20, 20 'at one end of the detection element 12 (shown in FIG. 5a) are electrically coupled to the point C of the bridge circuit 30 of FIG.
(Not shown) is electrically coupled to point B of the bridge circuit of FIG. Connector 2 at one end of correction element 14
2, 22 '(shown in FIG. 5a) are electrically coupled via a capacitor 36 to point D of the bridge circuit of FIG. 6, and the other end of the correction element 14 is coupled to connectors 23, 23'. 23 and 23 'are connected to the point C of the bridge circuit of FIG.

As shown in FIG. 6, a DC power supply 32 provides a heating current I to a detection element 12 in a vacuum environment 34. The capacitor 36 supplies the current from the power supply 32 to R2 or R2.
3. Means for preventing flow to R4. Therefore, unlike the conventional Pirani gauge using the conventional Wheatstone bridge, the heating current or the heating voltage of RS does not flow through R2.

The AC power supply 38 applies an AC signal voltage to the bridge 30 to generate AC signal currents i _s , i ₂ , i ₃ and i ₄ . Using these very small current values and the frequency selective detector 40, the bridge balance is detected,
The heat generation that occurs on the side of can be ignored. DC current I from the power source 32 is automatically calibrated by the controller 42, the AC voltage drop i _s RS AC voltage from point B to A from the point B to be measured by the AC voltage detecting function of frequency selective detector 40 to C It is always equal to the descent i ₄ R4. This automatic feedback linkage is indicated by broken lines 46 and 47.

The processor 51 comprises an ammeter 49 and a voltmeter 48
And a voltage drop of the detection element 12 and the detection element 12 based on the level of the heating current.
An output indicative of the pressure at 4 is produced.

Accordingly, the correction element 14 is
Operates at ambient temperature without pressure dependent electrical heating, even though it has the same physical size, thermal properties, and resistance properties.

Improvement 4 The fourth improvement is described with reference to FIG. This embodiment provides an improved apparatus and method for calibrating and operating the Pirani gauge of the present invention.

Unknown pressure P _{X at} bridge balance
Has been found to be accurate by a simple equation such as Equation 26.

[0139]

(Equation 26)

This finding differs from the conventional approach. It has been thought that the pressure indication depends not only on the resistance of the sensing element but also on other factors such as the ambient temperature. Therefore, conventional calibration methods often have to measure resistance and other values for calibration and operation. However, with the improvements described above, the VS and IS values provide sufficient temperature information for accurate pressure readings, eliminating the need to separately measure other parameters such as ambient temperature. Become. In this way, a three-dimensional calibration table that determines pressure based solely on voltage and current can be used.

To calibrate the gauge of FIG. 6, sensing element 12 is exposed to a series of representative known pressures and ambient temperatures over the pressure and temperature range of interest. Voltage drop VS _C and current IS _C are measured respectively by the voltmeter 48 and ammeter 49, are recorded along with the typical known calibration pressure P _C at bridge balance. These values may be recorded by a program running on the processor 51 or transferred to another processor unit for calibration calculations. The pressure P _C is the voltage VS _C and current IS
_Plotted against _C. The set of measurements at a given calibration temperature yields a temperature-invariant function relating pressure to voltage and current. As described above, these temperature-invariant functions are combined into one three-dimensional data table to define the calibration function of Equation 26. When this is done, the result is a set of points defining a surface, the height of which is the pressure, which is a function of the measured voltage and current.

The obtained calibration data is stored as a look-up table, and may be determined by interpolating the measured pressure from the pressure value stored in the look-up table based on the measured voltage drop and current. . However, because of the large number of points that must be stored in order to produce accurate output over a wide range of pressures, the present embodiment has obtained an approximate expression for determining the plane on which the measured values reside. This can be achieved by software that plots a three-dimensional surface. The resulting equation has the form shown in equation 26. To measure at any temperature unknown pressure P _X is, VS _X and IS _X are measured by the bridge balance, respectively, by voltmeter 48 and the ammeter 49. The correct pressure value is determined by permutation of Equation 26, giving Equation 27.

[0143]

[Equation 27]

For convenience, equation 27 is stored in processor 51 and is used to automatically calculate P _X when V S _X and I _X are input.

It will be appreciated by those skilled in the art that other amounts of voltage and current may be substituted within the scope of the present invention.
For example, a function of the form P = g (W, R) can be used instead of Equation 27, where W is the power applied to the detection element 12, and R is the resistance of the detection element 12. In this case, W and R can be calculated from the values of the voltmeter 48 and the ammeter 49. The important thing is 2
One selected parameter contains information about voltage and current, and the change in voltage and current is specifically reflected in a calibration graph or table based on the two parameter values. Thus, for example, the two input parameters of the function may be a group including any two of power, current, voltage, and resistance. In general, it is possible to find an expression of the form P = h (X, Y) that approximates the calibration plane. However,
X is the first input parameter, Y is the second input parameter, and P is the corresponding pressure. This equation is used as a substitute for a multi-dimensional calibration surface for calculating pressure.

With this improvement, excellent temperature correction from 10 ^-4 Torr or less to large pressure or more from 0 ° C. to 50 ° C. can be performed. There is no need to measure power and temperature as has been done frequently. U.S. Pat. No. 4,682,503
This improvement corrects for all types of errors caused by changes in the ambient temperature, such as radiation losses, as well as losses due to changes in the sensing element due to changes in the temperature of the wall as in FIG. This improvement avoids the complicated ambient temperature calibration using thermoelectric cooling described in US Pat. No. 5,437,869. In addition, the improved calibration and operating method automatically corrects for the fact that the temperature coefficient of the resistance of the sensing element at operating temperature is slightly different from that of the correction element at ambient temperature.

[Brief description of the drawings]

FIGS. 1a and 1b schematically show a conventional Pirani gauge.

FIG. 2 is a graph showing components of heat loss of a conventional Pirani gauge found by a study by the present inventors.

FIG. 3 is a diagram showing a conventional Pirani gauge using a small-diameter wire as a detection element.

FIG. 4a illustrates a portion of an improved heat loss gauge according to the present invention.

FIG. 4b is a sectional view of the portion shown in FIG. 4a;

FIG. 5a is an enlarged view of a cross-sectional view of both ends of an improved heat loss gauge according to the present invention, showing a coupling element and a support of a detection element and a correction element.

FIG. 5b is a cross-sectional view illustrating an embodiment of the mechanism of the present invention for maintaining the distance between the heat conducting plate and the sensing element and between the heat conducting plate and the compensating element.

FIG. 6 is a diagram showing a heating device dedicated to the detection element of the present invention.

[Explanation of symbols]

 10 Part of gauge 12 Detection element 14 Correction element 16 Plate 20 Detection element connector 22 Correction element connector 23 Connector 24 Electrical insulation film 26, 28 Spring 30 Wheatstone bridge 32 DC power supply 34 Vacuum environment 36 Capacitor 38 AC power supply 40 Frequency selection Detector 42 Controller 48 Voltmeter 49 Ammeter 51 Processor

Claims (24)

[Claims] A temperature sensitive sensing element; a temperature sensitive correction element; heating means coupled to the sensing element to supply power to the sensing element for warming the sensing element; Determining a gas pressure in an environment based on the temperature of the correction element, and measuring means coupled to the detection element and the correction element to generate an electrical signal indicative of the determined gas pressure; Is located at a distance from the detection element in the same plane as the detection element with substantially the same physical size as the detection element, and is made of the same material as the detection element. Heat loss gauge to measure in the environment. 2. The heat loss of claim 1, wherein said gauge further comprises first and second thermally conductive elements having first and second surfaces, respectively, near said sensing element. gauge. 3. The heat loss gauge according to claim 2, wherein said sensing element is 0.0002 inches to 0.002 inches away from said first and second surfaces. 4. The heat loss gauge according to claim 2, wherein distances from the detection element to the two surfaces are substantially equal. 5. The heat loss gauge according to claim 2, wherein the surface of the heat conductive element is made of a material having a heat conductivity of more than 0.25 Watt / cm / K. 6. The heat loss gauge according to claim 3, further comprising a unit for keeping a distance between the detection element and the surface constant regardless of a change in ambient temperature. 7. The heat loss gauge according to claim 6, wherein the means for maintaining the fixed distance includes a spring device for maintaining the tension of the detection element. 8. The bridge includes a bridge circuit including a first side including the detection element, a second side including the correction element, and third and fourth sides having the same impedance. The heat loss gauge according to claim 1, wherein the circuit includes means for preventing the power supplied to the detection element by the heating means from being supplied to the correction element. 9. A sensing element extending between the mounts at both ends and responsive to temperature, having a substantially uniform temperature zone between the mounts and a heat flow path between the mounts, and extending between the mounts at both ends. A compensation element responsive to temperature and having a substantially uniform temperature area between the mounts and a heat flow path between the mounts, coupled to the sensing element to supply power to the sensing element to warm the sensing element Heating means coupled to the detection element and the correction element to determine a gas pressure in an environment based on the temperature of the detection element and the correction element and to output an electrical signal indicative of the determined gas pressure; Heat means for measuring gas pressure in an environment, wherein the thermal conductivity and the physical size of the heat flow path of the detecting element are the same as that of the correcting element. Gauge. 10. A resistive detecting element having a resistance that changes according to an ambient temperature change, a correction element having a resistance that changes according to an ambient temperature change through a current, and a resistance of the detecting element at an ambient temperature. Heating means coupled to the sensing element for warming the sensing element to produce a constant difference between the resistances of the correction elements; anda gas pressure in an environment based on the relative temperatures of the detection element and the correction element. Measuring means coupled to the sensing element and the correction element for outputting an electrical signal indicative of the determined and determined gas pressure, wherein the current flowing through the correction element is substantially pressure sensitive. A heat loss gauge that measures the pressure of a gas in an environment, characterized by the absence of heat. 11. A resistive detecting element having a resistance that changes according to an ambient temperature change, a correction element having a resistance that changes according to an ambient temperature change, and a heating element for supplying a heating voltage to the detecting element. Heating means coupled to the sensing element; determining the gas pressure in an environment based on the relative temperatures of the sensing element and the correction element; and detecting the gas pressure to output an electrical signal indicative of the determined gas pressure. An element and measuring means coupled to the correction element, wherein the heating means is provided between the resistance of the detection element and the resistance of the correction element at ambient temperature without supplying a heating voltage to the correction element. A heat loss gauge that measures the pressure of a gas in an environment characterized by operating to create a constant difference. 12. The heat loss gauge according to claim 11, wherein the heating voltage is a DC voltage. 13. A resistive detecting element having a resistance that varies according to an ambient temperature change, a correction element having a resistance that varies according to an ambient temperature change through a current, and a heating current supplied to the detecting element. Heating means coupled to the sensing element for supplying; determining a gas pressure in an environment based on the relative temperatures of the sensing element and the correction element; and outputting an electrical signal indicative of the determined gas pressure. Measuring means coupled to the detection element and the correction element, the heating means including a resistance of the detection element and an ambient temperature without supplying a heating current to the correction element. A heat loss gauge that measures the pressure of a gas in an environment characterized by operating to create a constant difference between resistances. 14. The heat loss gauge according to claim 13, wherein the heating current is a direct current. 15. A typical voltage value required to: (1) subject the gauge to a known representative series of pressures; and (2) maintain the sensing element of the gauge at a desired temperature for each of said pressures. And (3) repeating the steps in (1) and (2) at an ambient temperature selected such that the calibration data defines a point on at least a three-dimensional calibration surface; 4) calculate the unknown pressure using the recorded calibration data by measuring the voltage and current required to warm the sensing element to the desired temperature when the sensing element is exposed to an unknown pressure; Determining the pressure corresponding to the voltage and current based on the temperature of the Pirani gauge. 16. Recognizing an expression of the form P = f (V, I) that approximates the surface when P is a pressure corresponding to a voltage V and a current I, wherein P is a pressure 16. The method according to claim 15, wherein the method is used in step (4) as a substitute for a calibration surface for calculation. 17. The method of claim 15, wherein values proportional to said voltage and current are used to represent said voltage and current. 18. A method comprising: (1) selecting different first and second parameters from the group consisting of heating power, current, voltage, and resistance; and (2) a representative series of known parameters at a known ambient temperature. Exposing the gauge under pressure; (3) recording the value of the first parameter necessary to maintain the sensing element of the gauge at a desired temperature for each of the known pressures at the known ambient temperature; (4) recording the value of the second parameter of the sensing element of the gauge at each of the known pressures and the value of the first parameter; (5) a multidimensional function that is a function of the first and second parameters. Steps (2), (3) and (4) are repeated at a plurality of ambient temperatures to define a pressure calibration plane; (6) when the sensing element and the correction element are exposed to an unknown pressure, Measuring the first and second parameters necessary to warm the output element to a desired temperature and determining the pressure corresponding to the first and second parameters based on a multi-dimensional calibration surface to determine the unknown pressure. Using the stored calibration data to calculate a method of temperature compensation in a Pirani gauge with a sensing element. 19. The method of claim 18, wherein said first parameter is heating power and said second parameter is resistance. 20. When P is a pressure corresponding to the heating power W and the resistance R, P = g approximating the calibration surface.
Recognizing an expression of the form (W, R), wherein said expression is used in said step (6) as a substitute for a multidimensional calibration surface for pressure calculations. The method described in. 21. The method of claim 18, wherein one of the first and second parameters is a heating power, and a value proportional to the power is used to represent this power. Method. 22. The method of claim 18, wherein one of the first and second parameters is a resistance, and a value proportional to the resistance is used to represent the resistance. 23. When X is the first parameter, Y is the second parameter, and P is a pressure corresponding to the values of X and Y, P = h (X, Y) approximating the calibration surface Recognizing an expression of the form: wherein the expression is used in step (6) as a substitute for a multidimensional calibration surface for pressure calculations. 24. A first side having a first temperature-sensitive resistive element, a second side having a second temperature-sensitive resistive element, and a third side having a third fixed resistive element. A bridge circuit including a side and a fourth side having a fourth fixed resistance element; and supplying an adjustable voltage between the first and second elements and between the third and fourth elements. Voltage means; and heating means coupled to the first element for providing power to the first element for warming the first element, wherein the second means for a given temperature. The temperature coefficient of the element
At a given temperature multiplied by the temperature, the resistance of the second element multiplied by the resistance of the fourth fixed resistance element is the temperature coefficient of the first element with respect to the given temperature. Is substantially equal to the value obtained by multiplying the resistance value of the first element by the resistance value of the third fixed resistance element, and the bridge circuit is connected to the first element by heating means. A heat loss pressure gauge comprising means for preventing the provided power from being provided to the second element.